Add two new test cases

gcc/testsuite/ChangeLog:

	* rust/compile/negative_impls.rs: New test.
	* rust/compile/negative_impls_2.rs: New test.

gcc/testsuite/rust/compile/negative_impls.rs

@@ -0,0 +1,8 @@
+#![feature(negative_impls)]
+
+trait ExampleTrait {}
+
+impl !ExampleTrait for i32 {}
+
+
+fn main() {}

gcc/testsuite/rust/compile/negative_impls_2.rs

@@ -0,0 +1,16 @@
+// This test case should error out since we haven't included the negative_impls feature
+// Output from  online rust compiler 2021 ver
+// Compiling playground v0.0.1 (/playground)
+// error[E0658]: negative trait bounds are not yet fully implemented; use marker types for now
+//  --> src/main.rs:8:6
+//   |
+// 8 | impl !ExampleTrait for i32 {}//
+//   |      ^^^^^^^^^^^^^
+//   |
+//   = note: see issue #68318 <https://github.com/rust-lang/rust/issues/68318> for more information
+
+// For more information about this error, try `rustc --explain E0658`.
+// error: could not compile `playground` (bin "playground") due to 1 previous error
+trait ExampleTrait {}
+
+impl !ExampleTrait for i32 {} // { dg-error "negative_impls are not yet implemented" }